What Have We Here?
Identifying the Condition of Materials in Old Buildings
Sooner or later most architects and engineers are faced with old building renovations, restorations, or additions. Frequently, the original documents and "as-built" revisions are lost. In this instance, two main challenges prevail:
- Determining the material properties of the concrete.
- Analyzing the reinforcing bars for yield strength, size, location, depth, bending, cut-off details and development lengths (bond and anchorage).
Reinforcing Bars
The American Society for Testing and Materials (ASTM) adopted the first standard specifications for billet-steel concrete reinforcing bars in 1911. Bars were designated as plain or deformed in structural, intermediate and hard grades (minimum yield strengths) of steel, or deformed, cold-twisted steel. Specifications for rail-steel and axle-steel reinforcing bars were also issued. Table 1 from CRSI's Evaluation of Reinforcing Bars in Old Reinforced Concrete Structures provides an excellent summary of 1911 to present day ASTM specifications, minimum yield and tensile strengths in psi.
For information on conversion from inch-pound to metric measurements, visit Metrication.
Bond and Anchorage
As of 1947, USA steel mills produced "high-bond" deformations which have continued almost unchanged to the present day. ASTM also issued specifications for deformations in 1947.
For older structures, consider all varieties of reinforcing bars as 50 percent less effective in bond and anchorage as with nondeformed bars bars. In other words, the tension development lengths for old bars is double that required for modern reinforcing bars.
Since most reviews for flexure strength will be based on a yield strength of 33,000 psi instead of today's 60,000 psi, the tension development lengths for the nondeformed bars can be determined by adding 10 percent to a current table of tension development lengths for today's reinforcing bars. The main deficiencies in old structures will be in tension lap splice lengths provided for bars larger than #6.
Details of Reinforcing Bars
Flexural Members
Between 1900 – 1940, many patented systems were applied to detailing and reinforcing bars that are now ineffective by today's standards. ACI Detailing Manuals for Buildings were first available in 1947. During evaluation of an old building, initial calculations can be based on the Manuals, but should be confirmed or modified as soon as data on bar sizes, bar spacings, and effective depths of structural members can be checked in the field.
For flexural members, load tests can be used to check calculated capacity based upon material tests and reconstituted placing drawings.
Columns
Non-destructive surface tests in multiple locations should be used to evaluate concrete. If necessary, remove the column concrete cover to observe vertical bar sizes, splice details, ties or spirals, etc. Load tests are usually not feasible, so determining column strength must be analytical.
Locating Reinforcing Bars
Instruments are available to locate reinforcing bars inside concrete. It is still advisable to expose bars at some non-critical location for calibration.
Concrete Properties
Testing of in-place concrete is important, even if the original project specifications are available. Specified concrete compressive strength is not reliable years after construction. To evaluate present in-place concrete strength, the following ASTM standard tests methods may be applied in correlation with strength tests on drilled cores:
- Test of cast-in-place cylinders
- Pulse velocity testing
- Rebound number
- Penetration resistance
- Pullout strength
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Evaluation of Reinforcing Bars in Old Reinforced Concrete Structures
(Engineering Data Report #48) |
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Evaluation of Reinforcing Steel Systems in Old Structures |
